The present invention relates generally to melter kettles that are designed and used to melt thermoplastic materials that are applied to pavements such as roadways, airport runways, parking lots, bicycle paths and other surfaces requiring pavement markings. More particularly the present invention is directed to systems and methods to improve the melting efficiency of melter kettles.
A variety of thermoplastic materials and compositions have been developed and used in the roadway striping industry. In order to apply such thermoplastic materials and compositions, they have to be melted and mixed. Melting, which involves both initial melting from solid stock or feed materials and maintaining the materials/compositions in a molten state for application onto roadways and other pavements, is typically conducted in melter kettles (also referred to herein as “melting kettles”) which can be heated by electrical means, or by burning combustible fuels.
Thermoplastic materials/compositions are the current products of choice for many types of marking applications. However, unlike most other types of marking materials thermoplastic materials/compositions must be melted for use. Thermoplastic materials/compositions can be applied by various methods such as spraying, extruding, and screeding. In order to be applied to pavement surfaces the thermoplastic materials/compositions need to be melted and heated to a sufficiently high temperature so as to adjust their viscosity as needed for a particular type of application process. In addition the temperature has to be controlled to avoid scorching, cooking, baking or breaking down.
Thermoplastic materials/compositions must be melted to very high temperatures that can reach up to 400° F. in order to be fluid enough to be applied using current pavement marking equipment. Early types of thermoplastic application equipment applied thermoplastic at slow rates. Therefore, long thermoplastic melting times required in the past to melt thermoplastic materials/compositions in melter kettles were not a problem. Melter kettles could keep up with low output application equipment.
Over time improvements in melter kettle designs were developed which reduced melting times. Eventually improvements in application equipment were developed which enabled thermoplastic materials to be applied at much faster rates. Soon it was recognized that the rate of melting thermoplastic in kettles was not keeping up with improvements in application equipment that increased the rate at which the thermoplastic material can be applied. While methods of application and equipment development have increased, the rate of application production melting capacity has lagged far behind the ability to apply the material.
For some time heat domes, also called heat risers or heat tubes, have been installed in melter kettles. A heat dome is formed by attaching a tube of variable diameter to a hole in the base of a kettle where the OD of the dome base matches the ID of the hole in the base of the kettle. The top of the dome is closed by a metal disc. The dome reduces the heating surface area of the base of the kettle; however, the dome provides additional circumference surface area that compensates for the loss of the heating area in a melter kettle with no dome within a few inches of dome height. Heat domes increase the heated surface area of melter kettles that is in contact with thermoplastic materials as compared to melter kettles that do not have heat domes thereby increasing the heat transfer into the thermoplastic materials in the kettles. This increases the ratio of heat transfer area to thermoplastic volume which improves heating efficiency.
An additional advantage of heat domes is that they provide for heating thermoplastic materials from the center of a melter kettle. Heating thermoplastic material in a melter kettle from the center of the kettle in an outwardly direction is more efficient than heat transfer from the outside of the kettle in an inward direction.
The use of heat domes in melter kettles has reduced melting times in kettles. However, heated air in heat domes cools as heat is transferred through the dome wall and top into the thermoplastic material being heated. This phenomenon limits the efficiency of heat domes. While melting times are reduced with the use of domes, further improvement is desirable.
The present inventor has recently developed a heat dome temperature regulating system that improves the melting efficiency of heat domes in melter kettles. The system, the subject matter of a copending patent application, includes a heat dome chimney stack tube that is attached to the top center of the heat dome around which an agitator drive shaft tube rotates. Heat travels from the heat dome up the center of the heat dome chimney stack tube and vents out of a top tube drive shaft heat chamber that is provided with an adjustable venting arrangement. This system exhausts air from the heat dome that has been heat depleted thereby allowing a continual flow of air heated to its maximum efficient temperature into the dome such that the maximum amount of heat is transferred through the heat dome and through the surfaces of the heat dome chimney stack tube into the thermoplastic material in the melter kettle. In this system the heat dome chimney stack tube and rotational drive shaft become heating surfaces that extend through the centerline of the kettle.
The present invention further increases the efficiency of melting thermoplastic materials in melter kettles.